Mouse Ovaries Boast Hidden Reserves

The long-held view that female mammals are born with their lifetime's complement of egg cells may not be correct after all. Research published in Nature (Vol. 428, No. 6979, pp. 145-150) shows that female mice continue to produce new egg cells after birth, casting doubt on a theory that has persisted for more than half a century.

The discovery hints that women, like men, may continue to replenish their reproductive cells during life, say Jonathan Tilly and colleagues. The researchers treated prepubertal female mice with a chemical that kills egg cells, and found that the mice still produced viable eggs in adulthood, showing that they can generate fresh eggs to replace damaged ones.

If the same process occurs in humans, it may shed light on why women's fertility declines after 30, says Allan Spradling in an accompanying News and Views article. Experts had previously been puzzled as to why eggs would begin to fail after 30 years instead of, say, 40 or 50. The new study suggests that this might be due to depletion of these 'replacement' reproductive cells, rather than defects in egg cells produced before birth.